Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. In particular, a common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in a database to determine proper identification of a person, such as for verification purposes.
In recent years it has been practical and economical to build high-quality electronic fingerprint sensing devices using radio-frequency (RF) electric fields to develop an electronic representation of the fingerprint pattern. Such devices have been fabricated as standard CMOS integrated circuits on monocrystalline silicon substrates. These processes allow the electronic structures necessary to read the signal from each of the sensor's pixels or sensing electrodes to be fabricated directly beneath the pixels. Locating the signal conditioning electronics or sense amps under pixel was important to adequate performance of the circuitry.
One such RF fingerprint sensing device is disclosed in U.S. Pat. No. 5,940,526 to Setlak et al. and assigned to the assignee of the present invention. The patent discloses an integrated circuit fingerprint sensor including an array of RF sensing electrodes to provide an accurate image of the fingerprint friction ridges and valleys. More particularly, the RF sensing permits imaging of live tissue just below the surface of the skin to reduce spoofing, for example. The entire contents of the Setlak et al. patent are incorporated herein by reference.
Another example of a fingerprint sensing device is disclosed in U.S. Pat. No. 5,325,442 to Knapp. The fingerprint sensing device has a row/column array of sense elements which are coupled to a drive circuit and a sense circuit by sets of row and column conductors, respectively. The sense elements are actively addressable by the drive circuit. Each sense element includes a sense electrode and a switching device, such as a thin film transistor (TFT) switching device, for active addressing of that sense electrode. The sense electrodes are covered by an insulating material and are for receiving a finger. Capacitances resulting from individual finger surface portions in combination with sense electrodes are sensed by the sense circuit by applying a potential to the sense electrodes and measuring charging characteristics.
Historically, electronic integrated circuits generally achieve reduced fabrication costs by using fabrication processes with smaller electronic device geometries. With smaller device geometries the circuit itself becomes smaller, using less silicon, and thus costs less to fabricate. Electronic fingerprint sensors, however, generally cannot be made smaller than the area of the finger skin that needs to be imaged. Smaller component geometries may not reduce the fingerprint sensor die size or cost significantly. One of the only results of smaller component geometries may be unused silicon space under the sensor pixels.
One approach to reducing the cost of fingerprint sensing is to design systems that can work effectively using images of smaller areas of skin. This approach has been used in a variety of devices. A second approach is to use sliding sensors. With sliding sensors, either the finger or the sensor move during the data acquisition process, which allows a small sensor to generate images of larger pieces of skin. Yet, the sliding sensors may be subject to significant image distortion, and/or they may provide an inconvenient user paradigm.
Image distortion or noise may also be present when it may be desirable to sense a finger image from fingers positioned at a relatively greater distance away from an array of finger sensing pixels. For example, it may be desirable to extend the range of electric field based finger sensors so that they can image fingers through significantly thicker dielectric materials such as molded plastic structures.
Integrated circuit based fingerprint sensors used in personal electronic devices, such as, for example, laptop computers and cellular telephones may be generally effective sensing fingers for user identity authentication. However, these finger sensors generally require the finger to be placed in a very close proximity to the array of finger sensing pixels. The need for close proximity to the user's finger typically makes the mechanical packaging and the integration of these devices into their host products more difficult and more costly.
Some approaches to address integrated circuit packaging of these devices include a special molded package with an opening in the molding allowing the finger a close approach to the array, and a specialized packaging on thin flex circuit substrates that allows the finger to be placed close to the sensing array, for example. Specialized packaging increases the cost of these sensors.
The need for the user's finger to be close to the array of finger sensing pixels may limit the thickness of the material that can be placed over that array, which may increasingly restrict the use of both protective and cosmetic coatings over the array area. Mechanically integrating these sensors into their host devices typically requires that the finger sensor project through a hole in the host device's case, so that the array of finger sensing pixels can be located about flush with the external surface of the case.
However, a negative aspect of these mounting arrangements may include increased difficulty and cost to seal the opening for the finger sensor in the host device case against the ingress of moisture, dust, and other contaminants. Additionally, the opening in the case with the finger sensor protruding may be cosmetically unacceptable to the intended appearance of the host device, and mounting the finger sensor in an opening in the case may be difficult and costly in some devices.